Пожалуйста, см. объяснение ниже. Актуальны 4й и 5й пункты.

Article usage

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...you have to understand two concepts: countability and definiteness. These concepts are explained in detail below.

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Five Sources of Definiteness
There are five principal ways in which a reader/listener can know specifically what a noun is referring to (that is, five reasons a noun might be considered definite):

The noun has been previously mentioned.
I saw a funny-looking dog yesterday [first mention, indefinite]. It looked like a cross between a Pekinese and a German shepherd. When it saw my cat, the dog ran away [second mention, definite].
A superlative or ranking adjective makes the noun's identity specific.
The tallest girl in the class is 6'2" tall. [There can be only one girl who is the tallest.]
Please read the fourth paragraph on page 3. [There can be only one fourth paragraph.]
Today is the most important day of my life. [There can be only one day that is the most important.]
The noun describes a unique person, place, or thing.
The earth revolves around the sun once every 365 days. [There is only one earth and only one sun--in our solar system, that is!]
A modifying word, phrase, or clause follows the noun and makes it clear which specific person or thing you are referring to. But not every noun that is modified in this way is definite; it depends largely on the situation and on what you can reasonably expect your listener/ reader to know about.
Do you remember the girl who went camping with us? [Using the here implies that there was only one girl who went camping with you; otherwise the clause who went camping with us would not be sufficient to identify the particular girl that you are referring to. If there were two girls, then you would have to be more specific, saying perhaps "Do you remember the girl from Iowa who went camping with us last May?"]
John is reading a book about quantum physics. [Here the noun book is modified by the phrase about quantum physics. But there is undoubtedly more than one book about quantum physics. Therefore, to make book definite, we would have to add more information: "the book about quantum physics that was assigned by Professor Jackson last week."]
The context or situation makes the noun's identity clear. For example, you might ask someone to "Close the door." You would use the because it would undoubtedly be clear to both of you which door you were referring to. Similarly, if you tell someone that you are going to the library, that person will assume that you are talking about whichever library is most familiar to both of you--RPI's Folsom Library, for example.
Again, you have to be sure that your reader or listener has the same context or situation in mind that you are thinking of; otherwise, he or she will be confused by your use of the. For example, one student wrote the following sentence:

This magazine helps women analyze the problematic situation and offers possible remedies.
But this was the first time she had mentioned a problematic situation. Her readers were therefore confused because her use of the word the implied that they were already supposed to know which problematic situation she was referring to.

http://www.rpi.edu/web/writingcenter/esl.html

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Note added at 5 hrs 50 mins (2005-04-18 06:07:01 GMT)
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Нумерация съелась :(
4й пункт начинается со слов \"A modifying word, phrase or clause follows\".
5й пункт начинается со слов \"The context or situation makes the noun\'s identity clear\"